The Transient Cavity Flow

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Selçuk Atalay (2010) The transient cavity flow. MSc by research thesis.

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UNIVERSITY OF GLASGOW

The Transient Cavity Flow

Selçuk Atalay

A thesis submitted in fulfilment of the requirements for the

Degree of Master of Science by Research

Department of Aerospace Engineering

Faculty of Engineering

September 2010

© Selçuk Atalay 2010

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Abstract

The use of cavity has many problems related to the application of weapons storage mechanisms, as the

opening of doors at subsonic and supersonic speeds produces high intensity noise which could damage

the internal stores and surrounding cavity structures. Furthermore, the flow inside the cavity could cause a

considerable drag force on the object.

The objective of this thesis is to explore experimentally the effect of the door kinematics in the transient

phase of the cavity flow experimentally. Transient wall pressure at 12 pressure taps which are arranged in

a line along the centreline of the cavity floor are measured in a very low-speed wind tunnel facility. Data

were acquired for two opening door mechanisms (One mechanism has a vertical doors opening

mechanism with a longitudinal hinge, and the other mechanism is a lateral sliding opening one). In both

cases, the door movement has been observed to affect cavity pressures over a long period of time, before

reaching steady levels. A passive flow control (cylinder) was also demonstrated to act effectively in the

shortening of the transient phase. These studies have been supported by flow visualisation, and a

preliminary use of Particle Image Velocimetry (PIV).

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Acknowledgements

First and foremost, I would like to thank my supervisor, Dr. Emmanuel Benard, for his

continuous support and advice throughout the project. His enthusiasm, guidance, hard work and,

most importantly, his belief and patience in me, has been tremendous help.

As always, my sincerest and humblest thanks go to my family. They have provided me a unique

sort of motivation and support over the last few years.

I would also like to thank the technicians whom support me during the laboratory facilities

especially Mr. Neil Owen, Mr. Tony Smedley, Mr Robert Gilmour and to other technicians who

support me.

Finally, I would like to thank my friends, Mr. Mazhar and Mesut and Adnan Altunkaya, Mr

Nadiir Bheekhun, and Mr Michea Giuni, for their constant presence during my research and life

in Glasgow, making it fun and enjoyable.

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Content

Abstract .......................................................................................................................................... iii

Acknowledgements ........................................................................................................................ iv

Chapter 1 Introduction .................................................................................................................... 2

Chapter 2 Literature Review ........................................................................................................... 5

2.1 Experimental Studies............................................................................................................. 5

2.1.1 Characterisation of Cavity Flow Types .......................................................................... 6

2.1.2 Description of Closed and Transitional Cavity Flow ................................................... 10

2.1.3 Description of Open Cavity Flow ................................................................................. 11

2.2 Flow Unsteadiness in Open Cavities ................................................................................... 13

2.3 Flow Control Studies ........................................................................................................... 15

Chapter 3 Experimental Set-Up .................................................................................................... 17

3.1 Introduction ......................................................................................................................... 17

3.2 Wind Tunnel Facility .......................................................................................................... 18

3.2.1Argyll Wind Tunnel....................................................................................................... 18

3.2.2 Flow Visualisation Low-Speed Wind Tunnel .............................................................. 19

3.3 Instrumentation.................................................................................................................... 19

3.3.1 Hot-Wire Anemometry ................................................................................................. 19

3.3.2Pitot-Tube ...................................................................................................................... 20

3.3.3.Particle Image Velocimetry (PIV) Experiment ............................................................ 20

3.3.4 Calibration of Instrumentations .................................................................................... 22

3.3.5 The Traverse System for Hot-Wire Probe .................................................................... 22

3.4 Models ................................................................................................................................. 23

3.4.1 Cavity Design ............................................................................................................... 23

3.4.2 The Doors Opening Mechanisms ................................................................................. 24

3.4.2.1 Opening Time ........................................................................................................ 24

3.4.2.2 Vertical Opening Mechanism ................................................................................ 25

3.4.2.3 Sliding Opening Door Mechanism ........................................................................ 28

3.4.3 Pressure Measurements ................................................................................................ 29

3.4.4 Passive Flow Control Experiment ................................................................................ 31

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Chapter 4 Experimental Results and Discussions ......................................................................... 33

4.1 Incoming Flow Characterization ......................................................................................... 33

4.2 Flow Visualisations ............................................................................................................. 34

4.2.1 Open Cavity Flow Visualisation................................................................................... 34

4.2.2 Flow Visualisations of Door Opening Sequences ........................................................ 36

4.2.2.1 4 seconds Door Opening ........................................................................................ 37

4.2.2.2 10 Seconds Doors Opening .................................................................................... 40

4.3 Unsteady Pressure Measurements ....................................................................................... 43

4.4 Flow Control Experiment .................................................................................................... 50

4.5 Repeatability Tests .............................................................................................................. 52

4.6 Preliminary PIV Results ...................................................................................................... 53

Chapter 5 Conclusion & Future Work .......................................................................................... 55

Chapter 6 References .................................................................................................................... 56

Appendix ....................................................................................................................................... 61

Appendix A. Calibrations .......................................................................................................... 61

A1 Instrumentation Calibration ............................................................................................. 61

A1.1 Pitot Tube Calibration ............................................................................................... 61

A1.2 Hot-Wire Calibration ................................................................................................ 62

Appendix B. Software ............................................................................................................... 63

B1 Labview............................................................................................................................ 63

B2 Calculations on Matlab .................................................................................................... 64

Appendix C. Drawings for Cavity............................................................................................. 66

Appendix D. Experimental Results ........................................................................................... 72

D.1.1 Repeatability Tests....................................................................................................... 72

D1.2 Opening Door till from 00 to 90

o degrees. ................................................................ 77


o degrees. ................................................................ 79


o degrees. ................................................................ 81

D1.4 Sliding Opening Door ............................................................................................... 83

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List of Figures

Figure 1 Illustrations of the bomb bays[34] ................................................................................................. 3

Figure 2 Typical cavity floor distributions for different types of cavity flow at subsonic and transonic

speeds. Reproduced with modifications in ESDU data-sheet 02008,[2] originally from Plentovich et al.

[3] Sub-figures correspond to: (a) open flow, (b) open/transitional flow boundary, (c) transitional flow,

(d) transitional/closed .................................................................................................................................... 6

Figure 3 Closed cavity flow for subsonic and supersonic speeds [30] ....................................................... 10

Figure 4 Open cavity flow for subsonic and supersonic speeds [30] .......................................................... 11

Figure 5 Typical noise spectrum inside the cavity. The acoustical signature is composed of narrowband

noise superimposed on top of broadband noise. Narrowband noise consists of discrete acoustic tones,

which are also referred to as Rossiter modes [21]. ..................................................................................... 13

Figure 6 Streamlines derived from the PIV velocity vector fields by Atvars et al. [18] (a) and Ukeiley and

Murray [19] (b). .......................................................................................................................................... 15

Figure 7 Cross-section of the Argyll Wind Tunnel and rolling road. ......................................................... 18

Figure 8 FC012 type Micro manometer ...................................................................................................... 20

Figure 9 Set-up of the PIV experiment ....................................................................................................... 22

Figure 10 Laser visualisation through the cavity. ....................................................................................... 23

Figure 11 The smaller cavity scheme. ........................................................................................................ 24

Figure 12 Basic kinematic scheme of the vertical opening door mechanism. ............................................ 26

Figure 13Transmission of the motion from the motor to the cavity [34]. ................................................... 27

Figure 14 The cavity front view when cavity is at 90 degree [34].............................................................. 27

Figure 15 Sliding door mechanics was attached to the plate. .................................................................... 28

Figure 16 The sliding door system. ............................................................................................................. 29

Figure 17 Instrumentation layout for pressure measurements. ................................................................... 30

Figure 18 Pressure Probe System................................................................................................................ 30

Figure 19 Cavity geometry showing the position of the cylinder stick. ..................................................... 31

Figure 20 Turbulent Boundary layer survey. .............................................................................................. 33

Figure 21. The cavity filled with smoke and cavity doors are closed. ........................................................ 34

Figure 22 On the downstream, the smoke layer has moved completely in 10 secs after opening doors. ... 35

Figure 23 The investigation of the shear layer which causes pressure fluctuations. .................................. 35

Figure 24 The vortices on the cavity floor. ................................................................................................. 36

Figure 25 At t=0 sec, the cavity was closed and fully filled using smoke. ................................................. 37

Figure 26 First second of the opening ......................................................................................................... 37

Figure 27. Second seconds of the opening. ................................................................................................. 38

Figure 28.At t=3 seconds, the shear layer flows appeared .......................................................................... 38

Figure 29. At t=3.5 seconds, the shear layer is growing related to previous figure. ................................... 39

Figure 30 t=5secs ........................................................................................................................................ 39

Figure 31 t=5.325secs ................................................................................................................................. 40

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Figure 32 t=6secs ........................................................................................................................................ 40

Figure 33 t=1.2sec ....................................................................................................................................... 41

Figure 34 t=2sec. ......................................................................................................................................... 41

Figure 35 At t=4secs, the shear layer forms appeared. ............................................................................... 41

Figure 36 t=5secs ........................................................................................................................................ 42

Figure 37 t=7secs. ....................................................................................................................................... 42

Figure 38 t=8secs. ....................................................................................................................................... 42

Figure 39 t=9.1secs. .................................................................................................................................... 43

Figure 40 t=10secs. ..................................................................................................................................... 43

Figure 41 Opening Door from 0o to 90

o at x/L=0.55 and free-stream velocity was 2m/sec. ...................... 44






o at x/L=0.55 and free-stream velocity was changing. ................... 46


o at x/L=0.55 and free-stream velocity was changing. ................... 47

Figure 46 Sliding Opening Door at x/L=0.55, opening time is 4.5 seconds. .............................................. 48

Figure 47 Sliding Opening Door at x/L=0.55, opening time is 10 seconds. ............................................... 48

Figure 48 Vertical Opening Door at x/L=0.30. ........................................................................................... 49

Figure 49 Vertical Opening Door at x/L=0.80. ........................................................................................... 50

Figure 50 Opening Door with and without flow control equipment. .......................................................... 51

Figure 51 Fully Opened Door with and without flow control equipment. .................................................. 51

Figure 52 Repeatability test of the door opening. ....................................................................................... 52

Figure 53 Repeatability test of the door opening. ....................................................................................... 52

Figure 54 Streamlines from PIV, for fully open cavity without flow control. (L=320mm, 2m/sec) .......... 53

Figure 55 Streamlines from PIV, for fully open cavity with flow control (L=320 mm, 2 m/sec). ............. 54

2

Chapter 1 Introduction

The research of the flow over cavities connected with the usage of bombers’ bays weapon bays

began in 1940’s. English Electric and Boeing are the first military companies which were interested in the

flow over weapon bays experiments; and their findings were expected to reflect the impact of the cavity

on stores and sensitive apparatus stored in cavity.

The main idea of cavity research is to investigate the system of the flow in a cavity. The use of

cavity has many problems related to the application of weapons storage mechanisms, as the opening of

doors at subsonic and supersonic speeds produces high intensity noise which could damage the internal

stores and surrounding cavity structures. In contrast, the aircrafts, which do not have any internal storage

system, carry the equipment under bombers’ wings, and this creates additional drag and heating over the

aircraft. Moreover, the radar cross-section of the aircraft is also increased. In order to overcome the above

problems, the current engineering solutions are directed at the creation and implementation of the flow

control devices, which would be able to regulate the air flow over an internal storage and improve the

stealth quality of the aircraft.

(a) Boeing UCAV X-45

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(b) Manned Combat Aircraft B1-B

Figure 1 Illustrations of the bomb bays[34]

Cavity flow studying is also essential for many other aerodynamic problems. For instance, as the

landing gear is released, the noise which is produced by engine is lower than produced by undercarriage

wells. In the automobile industry, the instance of the cavity flow can be found such as sunroofs and open

windows and doors.

In order to better understand of the complexity of the cavity flow plenty studying have

been revealed since first experimental works and also using computational fluid dynamics

(CFD).

The aim of this project is to highlight the significance effect of the door mechanism in the

shear layer form and also effect on the pressure fluctuations in the cavity. By using different type

of cavity and different type of cavity opening mechanisms and also for taking data several

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different techniques have been used to understand the fluid dynamics of the flow. The literature

review is very tight for door opening mechanics. Thus, it could not be possible to compare our

results with the work has been done before and also the condition that has been work was not

comparable with others.

The outline of that thesis is as follows. Chapter 1 introduces the basic description of the

current work, cavity flow problems, and the motivation for the objectives. Chapter 2 presents the

literature review which includes the work done till now and their results and the methods that has

been used. Chapter 3 details the experimental methodology. The design of the cavity and

describes the equipment that used during the experiment. It also includes that the experimental

methods used and data acquisition systems. Chapter 4 is about experimental analysis and

discussion. It includes the data about different opening mechanism doors with various different

parameter and compares of them. And finally in Chapter 5, the conclusion and future prospects

are presented.

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Chapter 2 Literature Review

The prime objective of the literature review is an attempt to better understand works

connected with cavity flows and also identify from these works concerning about the basics

mechanics of the flow with depends on several parameters. For example, boundary layer, shear

layer instability, pressure gradient, acoustic radiation are all key parameters for cavity flow. All

works done regarding cavity flow is to clarify the exact nature of cavity flow mechanics.

2.1 Experimental Studies Plenty number of works have been done since early 1940’s by researchers. The

experimental works showed that the flow characteristic over the cavity is depending on several

parameters. The most important parameters are L/D ratio, free-stream Mach number, upstream

boundary layer thickness. The cavities were classified related to their geometry and divided into

two groups as deep and shallow. Deep cavities occur whether having L/D≤1 and shallow cavities

as having L/D>1. Though, investigation of the pressure distribution along the cavity floor has

showed that the flows over cavities can be grouped into three classes: open, closed and

transitional [1].

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2.1.1 Characterisation of Cavity Flow Types Open and closed flow cavities were first introduced by Rossiter and Charwat et al. Open

flow occurs for deep cavities as L/D ratio ≤10. Closed cavities which occur as L/D≥13 for

shallow cavities. However, defining the boundary between open and shallow cavities is not

straightforward. Recently, in order to define the boundary for transitional, Plentovich et al.

(1992) [2]defined the limits between deep and shallow as the open cavity for L/D is less than or

equal to 6-8, transitional for 7 ≤ L/D ≤ 14 and closed for L/D is greater than or equal to 9-15.

Figure 2 Typical cavity floor distributions for different types of cavity flow at subsonic and transonic speeds.

Reproduced with modifications in ESDU data-sheet 02008,[2] originally from Plentovich et al. [3] Sub-figures

correspond to: (a) open flow, (b) open/transitional flow boundary, (c) transitional flow, (d) transitional/closed

flow boundary, (e) closed flow, (f) closed flow. l/h denotes cavity length to depth ratio.

After several experiment, Plentovich et al [3] realised that the boundaries are not only

dependent on the L/D ratio, and also cavity width (W) and free-stream Mach number. He showed

that while Mach number and W/D ratio is changing, the limits between open and transitional

remained relatively constant. Moreover, the L/D ratio is increasing, while Mach number and

cavity W/D ratio is increased.

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8

9

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2.1.2 Description of Closed and Transitional Cavity Flow As the cavity L/D ratio is increased, the shear layer flow has not enough energy to span

across the cavity length. Hence, the shear layer flow attaches at some point along the bottom of

the cavity and reaches the trailing edge of the cavity. At the cavity front, the region is low

pressure forms and at the cavity rear, the region is high pressure forms. Acoustic tones cannot be

observed in closed cavity. The drag coefficients are higher comparing to the open cavity.

Transitional cavity flow happens as declared previously between the limits of open and

closed flow. Whether the cavity length is increased, the shear layer begins to detach from the

cavity floor. It is a form of closed cavity. In contrast, as the cavity length is decreased, the shear

layer begins to acts as open cavity. Consequently, reducing the L/D, the shear layer does not

impinge on the cavity floor, as well as pressure gradients are reduced.

Figure 3 Closed cavity flow for subsonic and supersonic speeds [30]

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2.1.3 Description of Open Cavity Flow Open cavity flow occurs for deep cavities. In this form, the flow separates from the

leading edge of the cavity and a shear layer flow bridges the length of the cavity and impinges on

the downstream of the cavity wall. That impingement generates acoustical disturbances that

propagate upstream as pressure waves. The other form of the shear layer circulates inside the

cavity and this circulation makes the static pressure distribution along the cavity floor nearly

uniform, at the downstream side of the cavity, there is slightly higher pressure level. This

homogeneous distribution is a necessary condition because it allows to safe store. Although, the

shear layer impinges with downstream wall of cavity, the cavity produces high intensive noise

which causes vibrations on both cavity and store.

Pressure waves which produced by high acoustic tones has been first found by

Krishnamutry [4]. He assumed that the source of the pressure waves has two sources; however,

Rossiter [5] predicted that it has only one single source which is downstream wall of the cavity.

Figure 4 Open cavity flow for subsonic and supersonic speeds [30]

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The characteristic property of open cavity is illustrated on the graph. The band range is

formed by broadband noise and narrow-band tones. The sources of the broadband noise are free-

stream Mach number, shear layer form and varied laminar and turbulent flows. Rossiter

developed a formula to calculate the acoustic frequencies analytically created in the cavity. After

that studying, tones names with Rossiter modes [6].

Rossiter’s Semi-Empirical Formula

Based on the empirical results, J.E Rossiter [6-10] developed a formula to calculate the cavity

resonance frequencies. He assumed that the those frequencies has an equivalent mth

mode was

given as

(1.1)

where f is the frequency, L is the cavity length, U∞ is free-stream Mach number and n is the

Strouhal number.

∞

(

) ∞

(1.2)

In formula (1.2), m is the mode number, and Kv are determined from experiments which are

respectively 0.25 and 0.57 for cavity type L/D ratio is 4 which is used in T.S.R.2 [5].

That formula can be arranged related to periodic time of the pressure fluctuations. That time is

the time which is required the travel the length of the cavity at half of the free-stream Mach

number and then go back to the cavity at sonic speed.

(1.3)

a∞ is the speed of sound.

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In addition, Rossiter observed the pressure fluctuations for several different cavities which have

various L/D ratios. He also focused on deep cavities in order to see the same tones; however, as

the L/D is increased, the pressure fluctuations inside the cavity are getting random forms and the

tones are getting wide range fluctuations. He extended the semi-empirical formula for supersonic

speeds.

Figure 5 Typical noise spectrum inside the cavity. The acoustical signature is composed of narrowband noise

superimposed on top of broadband noise. Narrowband noise consists of discrete acoustic tones, which are also

referred to as Rossiter modes [21].

2.2 Flow Unsteadiness in Open Cavities In order to understand the unsteadiness in open type cavities, it is necessary to analyse shear

layer flow. Heller and Bliss [11] investigated that the aft wall of the cavity is the source of

acoustics and front wall of cavity is reflecting wall.

According the curvature of the shear layer form, the impingement angle of the shear layer to the

aft wall of the cavity is taking form. Due to the curvature, the static pressure distribution along

the cavity cannot be balanced. Clearly, assuming that the shear layer is smooth along the cavity

gives a result that unbalanced pressure does not occur inside the cavity. Unsteadiness in the shear

layer is due to the unbalanced pressures.

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The interaction between trailing edge of the cavity and the shear layer generates acoustic waves.

In 1978, Rockwell and Naudascher [12, 13] postulated that the result of the interaction between

the shear layer and aft wall of the cavity produces acoustic waves. These waves then propagated

upstream and disturbed the coming shear layer at the cavity front wall. The interference at the

cavity front instigated further oscillations of the shear layer form, thereby completing the

feedback loop.

Again in 1978, the front cavity wall and cavity floor are also the source of the acoustic waves

because the pressure waves are reflecting from these parts of the cavity and interfere with shear

layer. It has been postulated by Tam and Block [14]. They studied based on the shear layer

oscillations and mass breathing process. Further experimental studies have been done with using

several techniques like Hot-Wire Anemometry (HWA), dynamic pressure transducers, Laser

Doppler Velocimetry (LDV), Particle Image Velocimetry (PIV), Schlieren Photography and

Shadowgraph. These techniques are useful to fully understand the flow mechanism inside the

cavity (especially near the walls) [15-16]. Each technique has own advantage to get data from

which depend on the flow interfere. For example, hot-wire are used to get data boundary layer

survey, dynamic pressure transducers are used to measure unsteady flow on the surface

especially side walls and bottom of the cavity. In order to classify the flow optical techniques

were used like Schlieren Photography and Shadowgraph [4, 6, and 17]. LDV and PIV are more

advanced related to other techniques. Their advantages are obtaining high-fidelity, high-

resolution data from the instantaneous flow-field and the velocity variations inside the cavity.

The below figures are from the two different PIV experiment results. That indicates mean

streamlines of the flow from two different experimental studies. The first figure from the Atvars

et al [18]. Its experimental setup is like cavity was surrounded by flat plate and L/D>1, the other

one from Ukeiley and Murray[19], the cavity L/D<1. For both cases, a large vortex has been

observed but their core places are in different places.

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Figure 6 Streamlines derived from the PIV velocity vector fields by Atvars et al. [18] (a) and Ukeiley and Murray

[19] (b).

2.3 Flow Control Studies In order to cover the occurring problems of the cavity, understanding of the cavity flow

mechanisms is needed to design flow control methods to reduce the acoustic tones inside the

cavity. The first experimental solutions were about the modification of the cavity geometry: For

instance; adding spoiler or modifying the angle of side walls or adding external equipment to

change the flow behaviour inside the cavity. These kinds solutions are called as passive flow

control and were used in 1950s by Norton (1952) [20] and Rossiter (1962) [21]. The use of

spoilers on the leading side of the cavity has been used to reduce the cavity rattling. The effect of

the spoiler on cavity environment was that the spoiler was increasing the boundary layer

thickness and Norton realised that the spoiler can modify the shear layer forms. Result of the test

has been shown that the bomb bay pressure fluctuation can be reduced [22].

The flow control works focus on supressing the high acoustic level noise caused by shear layer

mode. In that way, some works have been done by Shaw and Shimovetz [23]. They

recommended that shear layer providing that whether the shear layer can be managed to jump

over the cavity opening or attach at a point in the middle way of cavity opening, the feedback

process would not appeared and hence pressure waves will not occurred. The basis of the flow

control works depends on that principle. There are two kinds of control mechanism; passive and

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active control. In active control, the additional devices are needed as external energy sources in

order to change the flow within the cavity [24]. The external energy sources are jet blowing,

mass injection, and oscillating flaps. Active control (external energy input) can be divided into

open-loop and closed-loop control. For passive control studies, modifications of the cavity

geometry or extra physical devices can be added such as leading edge spoilers, inclined cavity

walls, pins [25], steps [26], transverse rods.[27]. The effectiveness of the rod to the spoiler has

proved that lifting the shear layer into the free stream does not change the pressure fluctuations

inside the cavity. The used spoiler was more convenient in order that lifting the shear layer; in

contrast, the rod was causing less pressure fluctuations at high speed free-stream velocities [28].

Shaw [29] proved that the cavity which has inclined trailing edge wall, supresses the pressure

fluctuations, acoustics waves and broadband noise at high speeds.

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Chapter 3 Experimental Set-Up

3.1 Introduction Based on the objectives set out at the onset of the research, a series of experiments, aimed at

gathering qualitative and quantitative information of the effect of the door mechanism on the

cavity flow, was performed. These experiments were designed to study wide areas of the door

mechanism effect and to determine the significance of the opening timing of the doors on the

fluid dynamics of the transient cavity flow. The experiments conducted produced both

qualitative images, visualising the influence of door mechanism on the cavity flow, and two-

dimensional velocity data from during the opening doors of the cavity and the effect of a cylinder

row on the cavity flow. Type of the opening doors, opening timing of doors, different opening

angles, adding additional flow control mechanism configurations were varied during these

experiments to determine their influence on the resulting cavity flow. The following sections of

this chapter describe the various experiments that were conducted during the course of this

research and provide details on the experimental facilities, cavity models, experimental

techniques and apparatus used during each experiment.

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3.2 Wind Tunnel Facility

3.2.1Argyll Wind Tunnel

The wind tunnel used for PIV experiments is the so-called Argyll wind tunnel. It is a closed

return tunnel with an octagonal cross-sectional working section, measured to have a width of

2.65 m and a height of 2.04 m, and a maximum flow velocity of some 72m/sec. A rolling road,

of 1.9m width and a length of 3.75m makes up the floor of the working section, with the rolling

road belt having a width of 1.63m. The rolling road is claimed to reach speeds comparable to the

speeds of the tunnel, although the tunnel can be used independently from the rolling road as

required. A diagrammatic representation of the wind tunnel working section and the rolling road

is shown in Figure 7

Figure 7 Cross-section of the Argyll Wind Tunnel and rolling road.

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3.2.2 Flow Visualisation Low-Speed Wind Tunnel The facility which is used to perform the investigation is a low speed open circuit wind tunnel

with a test section 910 mm wide, 915 mm tall and 4580 mm long. In the front part a contraction

takes place with a contraction ratio of about 9. Here a honeycomb and a wire screen are placed in

order to obtain a uniform flow entering the wind tunnel, in which the maximum speed reachable

is 2.7 m/s.

One side of the wind tunnel is made of perspex glass windows which allow having optical access

to the inside and this wind tunnel is supported by smoke generator, laser illumination with high

resolution digital image capture. These properties have been useful to observe the unsteady

boundary layer and the effect of the doors when they are moving. Also, to decide where the

pressure taps should be more often, the flow visualisation was the key.

It was available for the experiment a smoke injector which has been proven to be very useful in

the understanding of the features of the flow inside the cavity, especially through visualization of

the behaviour of the boundary layer. This permitted to observe with great detail the process of

transition of the boundary layer, with much more insight than quantitative hot wire measurement.

3.3 Instrumentation

3.3.1 Hot-Wire Anemometry

Boundary layer survey and velocity profiles of the shear layer were carried out using a hot wire

anemometry which was a DISA type 55M10 constant-temperature anemometer (CTA). Dantec

type device was connected to data acquisition card (DAQ) which was National Instrument USB-

6229, in order to collect the data simultaneously. The sampling rate for boundary layer survey

20

determined as 200 Hz and sample size was 65k. During the measurements a boundary layer type

probe was used.

3.3.2Pitot-Tube

A pitot tube were employed in order to calibrate the hot-wire and collect data from unsteady and

steady pressure measurements of the cavity bottom with a nominal sensitivity of ±1% of reading.

During the unsteady pressure measurement, the digital output from pitot tube was connected to

the daq card. Besides pitot tube is located along the longitudinal centreline of the settling

chamber to facilitate measurement of total pressure during the calibration of hot-wire and has

been settle under the cavity during capturing the unsteady and steady pressure difference. The

free-stream velocity of the wind tunnel has been measured by using pitot tube.

Figure 8 FC012 type Micro manometer

.

3.3.3.Particle Image Velocimetry (PIV) Experiment

Particle Image Velocimetry (PIV) is an optical experimental technique used to quantitatively

measure fluid flows. It utilises light reflecting properties of tracer particles suspended in a fluid

to measure its velocity. This method relies on the fundamental assumption that the tracer

particles suspended in the flow faithfully follows the fluid motion. In this technique, tracer

21

particles of nominal diameters between 1 - 2µm are used to seed the flow. A powerful light

source, usually a pulsed laser, is used to illuminate the required region of interest of the flow-

field.

The laser beam is normally expanded into a thin light sheet of 2-3mm thickness, and the laser is

pulsed to produce short, powerful bursts of light. In the case of two-dimensional two-component

(2D1C) PIV, the method employed for this preliminary investigation of the flow inside the

cavity, an imaging system is placed normal to the flow region of interest. Two successive images

of the particle seeded flow-field, separated by a known time delay, referred to as the inter-pulse

time delay, Δt, is recorded. Particle displacements between the two recorded images are derived

through correlation analysis. Images recorded during the PIV are subdivided into smaller

interrogation areas, and corresponding interrogation areas are usually cross-correlated to derive

the particle pixel displacement information. A calibration of the PIV system is done to relate the

pixel and physical coordinates of the flow-field and provide transfer functions necessary to

convert the derived pixel displacements to physical

particle displacements. With the inter-pulse time delay known, the velocity of the flow-field is

then derived from the images.

The equipment for PIV system based on a Spectra Physics Lab130-10 Nd: YAG single cavity,

double pulsed, frequency doubled laser, with a wavelength of 532nm and a Kodak Mega Plus

ES1.0 digital video camera. The laser was Q-switched, to produce short, high energy laser

pulses, with pulse duration of 8ns. The laser beam was expanded into a light sheet that was

aligned with the symmetry plane of the cavity, along the wind tunnel longitudinal axis.

Seeding was produced using a Concept Systems VI Count Smoke Generator that heats smoke oil

to produce a fine oil mist.

Flow over the cavity was imaged using a single camera. This allowed investigating streamwise

and vertical components of the velocity of test section. Image post-processing was carried out

using the Davis software.

22

Figure 9 Set-up of the PIV experiment

3.3.4 Calibration of Instrumentations

To obtain the velocity value from the output voltage given by the anemometer the probe has to

be calibrated. The air speed at which the investigation is performed is as low as 0.3 m/s, making

the calibration of the probe a critical task, because at such low speeds natural convection occurs

due to heat generated by the probe. The probe is calibrated directly in the wind tunnel by taking

at least 17 equally spaced points from 0.3 to 2.2 m/s, using a pitot tube connected to a digital

manometer with an accuracy of 0.01 m/s. Usually a 4th order polynomial curve is used for the

fitting of the V−E couples. Gain and offset are also adjusted to reach the full range of the A\D

board.

3.3.5 The Traverse System for Hot-Wire Probe

In order to move the hot-wire probe through the boundary layer thickness, PC controlled traverse

system has been used. This was placed above the wind tunnel, from where the probe support

reached the plate and also the cavity was exists. The motor of the hot-wire traverse was

controlling by NI instrument card and that card was controlling by Labview Signal Express

program which allows precisely steps of about 0.075 mm.

23

3.4 Models

This section is about design for the cavity model and modifications.

3.4.1 Cavity Design

There are two designed cavity model; both has the same characteristic L/D ratio. The cavity

model has shown in the figure. The cavity has been mounted to the wood block on a plate in the

wind tunnel. For the purpose of this project, open type cavity was employed. The dimensions of

the cavity is L/D=5. The bigger cavity length was identified as 750 mm with a depth and width

of 150 mm and smaller one has 320mm length and 64 mm depth and width.

Figure 10 Laser visualisation through the cavity.

24

Figure 11 The smaller cavity scheme.

3.4.2 The Doors Opening Mechanisms

3.4.2.1 Opening Time

The cavity has two type opening door mechanism; they are vertical opening door and sliding

opening door mechanisms.

The importance of the opening and closing door mechanism is to understand the effect of the

opening time on shear layer flow. Because as studied in literature review, the source of the high

acoustic noise is shear layer form on the trailing edge of the cavity. However, the current

experimental conditions are not enough to produce Rossiter frequencies as free-stream velocity is

very low.

The flow which travels from one edge of the cavity to another takes 0.375 sec. The Strouhal

number based on the resonance frequency which is given as;

Cavity Sliding Door

Cavity

25

(3.1)

fr is the resonance frequency, L is the length of the cavity and U is the free-stream velocity.

Considering the first resonance Strouhal number is 0.1 and the resonance frequency can be found

as 4secs from the formula (3.1). The frequency shows that when the cavity doors opened less

than 4 secs, the resonance frequency cannot be observed. The importance of the opening time

sequence of the cavity door is to prevent these high level oscillations inside the cavity.

3.4.2.2 Vertical Opening Mechanism

In order to approach to real aircraft properties, the vertical opening door mechanics has been

chosen. The example of the can be observed on B1 Bomber, B2, F-117, F-111, UCAV X-45 and

so on. The cavity working scheme has been designed previously. The power is to turn on

longitudinal axial, comes to the doors from shoulder via motor. A Panasonic type motor which

connected to shoulder, can be controlled by PC-based data acquisition card by servo motor

control device which has been designed by University of Glasgow technician (Neil Owen). The

vertical opening scheme has been shown in below figure.

26

Figure 12 Basic kinematic scheme of the vertical opening door mechanism.

The advantage of the controlling the door mechanics by control based DAQ card is to change the

position and opening time of the doors with desired position and velocity and via a channel from

servo control, the position of the door can be understood.

27

Figure 13Transmission of the motion from the motor to the cavity [34].

Figure 14 The cavity front view when cavity is at 90 degree [34].

28

3.4.2.3 Sliding Opening Door Mechanism

The sliding system has been designed in order to compare the results with CFD. Since the

generation of the mesh for vertical door opening case is complex related to the sliding door

mechanics.

Figure 15 Sliding door mechanics was attached to the plate.

The kinematic of the sliding is quite basic as the door which made from Perspex material as for

PIV experiments, is attached to the sliding system and can be move along cavity width on the

plate. The sliding motor was controlled by PC-based daq card. As a programme language, “M

“and “G “code has been used.

29

Figure 16 The sliding door system.

3.4.3 Pressure Measurements As shown in below figure, an array of pressure taps was drilled on the centreline of the cavity

floor plate. For the measurements of steady and unsteady pressure measurements, a pitot-static

probe was used. The pressure taps were aligned along the centreline of the cavity floor. Unsteady

pressure measurements were registered during the cavity doors were opening.

Sliding System Mechanism

Plate

Cavity Door

Direction of Door

30

Figure 17 Instrumentation layout for pressure measurements.

The certain locations of the pressure locations are at x/L=0.20, 0.30, 0.40, 0.45, 0.50, 0.55, 0.65,

0.70, 0.75, 0.80 and 0.90, respectively.

The pressure measurement was carried out using a system as shown in figure. The static pressure

probes was connecting to the taps under the cavity and measuring the pressure gradient related to

the total pressure probe which is on the free-stream and this process was repeated for all point for

steady and unsteady case.

Figure 18 Pressure Probe System.

Cavity centreline

y

x

Cavity Length

Free-stream velocity

31

3.4.4 Passive Flow Control Experiment

In order to prevent the oscillation inside the cavity, some flow control techniques have been tried

as researched in literature review. One of them was about locating a cylinder stick on the

upstream of the cavity and try to reduce the oscillations or unwanted pressure waves inside the

cavity. For that experiment, we used a cavity which length was 320mm and deep and width was

64mm.

Figure 19 Cavity geometry showing the position of the cylinder stick.

For pressure measurement, the length of leading edge of the cavity was 100 mm and the cylinder

stick was located 10mm away from the cavity upstream edge. The diameter of the cylinder was

chosen as half of the boundary layer thickness, which was 2.5 mm. The free-stream velocity was

approximately 18m/sec. In this experiment, amount of the flow which was interacting with the

cavity has been calculated by weight scale which was located under the cavity. Moreover, only

sliding opening door system has been used.

For the PIV experiment, the flow control experiment has been progressed, as well. However, in

this case the upstream leading edge plate has been extended to 550 mm in order to determine the

Length= 320mm

Depth= 64mm

Cylinder Stick Free-Stream Velocity Vector

Sliding Control

Mechanics

Cavity Door

32

boundary layer thickness as 50cm. additionally; the cylinder row has been situated just above the

cavity leading edge and the cylinder row was hanging on the air like 6mm..

33

Chapter 4 Experimental Results and Discussions

4.1 Incoming Flow Characterization The free-stream velocity has been determined as 2m/sec for that experiment. The boundary layer

was captured from 5 cm away from the upstream of the cavity leading edge and 50 far from

leading edge of the plate. In order to create fully turbulent boundary layer, row cylinder located

on the leading edge. The height of the boundary layer determined as 50mm.

Figure 20 Turbulent Boundary layer survey.

Before pressure measurement test inside the cavity, at several points on the cavity, shear layer

survey has been done using hot-wire anemometry. However, during the door opening, there is no

access for hot-wire to get some data.

34

Considering the literature review, in some cases, the interaction of the shear layer with

downstream corner of the cavity causes pressure fluctuation and that pressure performances as

acoustic sources [19]. Thus, it is decided that to measure the pressure differences inside the

cavity. The cavity flow mode could be characterized by analysing the pressure measurements of

the cavity floor. [31].

From the literature review, the places of the pressure tapping was trying to find out, however, no

one studied at very low speed to investigate the pressure fluctuations on the cavity floor. Using

the flow visualisation with tunnel, the flow behaviour inside the flow has been exposed.

4.2 Flow Visualisations

4.2.1 Open Cavity Flow Visualisation

In order to understand, at which position of the cavity floor, the pressure gradient is high, the

cavity filled with smoke completely as seen in figure 21.

Figure 21. The cavity filled with smoke and cavity doors are closed.

After opening the cavity doors, almost all smoke layer on the downstream of the cavity, has been

moved out by flow. Comparing to downstream, on the upstream, nearly the all smoke was

staying without any movements. The figure 22 shows that the flow on the downstream was

Free-Stream

35

considerable for pressure measurements, thus, more often pressure taps needs on the

downstream.

Figure 22 On the downstream, the smoke layer has moved completely in 10 secs after opening doors.

Figure 23 The investigation of the shear layer which causes pressure fluctuations.

Vortex

Shear Layer Form

36

Figure 24 The vortices on the cavity floor.

As a result from the flow visualisation of the cavity, it is decided that to locate the pressure taps

more often on the middle of the cavity. The produced vortices because of the shear layer form

have been seen clearly from captured movie sequences, and they were generally situated on the

middle of the cavity.

As shown by small black points in figure 17, the pressure taps located on the centreline of the

cavity floor as similar at Ukeiley et al. [19] Ziada et al. [32] and Daoud [31].

4.2.2 Flow Visualisations of Door Opening Sequences

As described in previous chapter, the cavity has vertical door opening system and one of the

unique features of that project is to have a vertical door opening system. The fluid dynamics of

aim of that project is to understand to how the door affects the flow inside the cavity.

During the experiments, several different opening time sequences have been tried. However, the

flow visualisation has been done only two time sequences, 4secs and 10 seconds.

Vortex

37

4.2.2.1 4 seconds Door Opening

In order to understand the changing of the flow inside the cavity, the cavity filled again

completely with smoke in figure 25.

Figure 25 At t=0 sec, the cavity was closed and fully filled using smoke.

The effect of the doors on the fully filled by smoke cavity started to seen as in figure 26. Based

on the cavity movement, some vortices have been created. Thus, such kind of vortex would not

appear comparing to the in 10 seconds opening doors. The vortices depend on the cavity doors

like when the doors were opening slowly in 10 secs, the vortices shape was quite smaller.

Figure 26 First second of the opening

Vortices

38

Figure 27. Second seconds of the opening.

In two seconds of the opening doors, the vortices cause high pressure gradient unexpectedly, this

peak can be observed in the Cp graphs. In three seconds, the shear layer forms were appeared.

The flow close to the cavity floor was travelling to reverse direction of the free-stream flow.

Figure 28.At t=3 seconds, the shear layer flows appeared

Shear layer forms

39

Figure 29. At t=3.5 seconds, the shear layer is growing related to previous figure.

With the movement of the doors, the open area was increasing and the penetrating flow to the

inside of the cavity was getting more, so it causes effective shear layer forms.

Figure 30 t=5secs

40

Figure 31 t=5.325secs

Figure 32 t=6secs

In the 5th

second the cavity opening has been completed. By the time, the coming flow was

sweeping the downstream of the cavity till the point at x/L=0.90. During the pressure

measurement, the unsteadiness at points x/L=0.70, 0.75, 0.80 and 0.90 was existed. However, in

the upstream of the cavity floor, the pressure gradient was nearly steady after opening. The

reason for unsteadiness in the downstream of the cavity was the interaction of the shear layer

flow with the aft wall causes pressure fluctuation and it was the source of the vortices [19].

4.2.2.2 10 Seconds Doors Opening

The flow visualisation technique could give some information about the effect of the cavity

opening time to the flow inside the cavity. Thus, 10 seconds opening time was also tried.

Distinctively, at 10 seconds opening, in first second, the effect of the flow was not very obvious

comparing to the 5 seconds opening. As the gap of the door was quite tight related to the five

seconds opening.

41

Figure 33 t=1.2sec

Figure 34 t=2sec.

After 4 seconds, the shear layer forms started to seen apparently.

Figure 35 At t=4secs, the shear layer forms appeared.

42

Figure 36 t=5secs

Figure 37 t=7secs.

The shear layer flow was sweeping the smoke in the downstream corner of the cavity.

Figure 38 t=8secs.

43

Figure 39 t=9.1secs.

Figure 40 t=10secs.

As seen in figures, the only changing related to the in five second opening door is the time delay.

The shear layer form grows with taking distance on the cavity, then it causes more pressure

fluctuations.

4.3 Unsteady Pressure Measurements The unsteady and steady pressure measurements have been recorded from 12 pressure taps

which were located on the centreline of the cavity floor. The steady pressure measurements have

been taken when the doors are fully open and closed and the unsteady measurements taken

during the doors are opening from 0o degree to 90

o degree or other variations.

44

Using the Cp formula;

(4.1)

Where P is the pressure coefficient where pressure evaluating, P∞ is the pressure in the free-

stream, is the free-stream fluid density and V is the free-stream velocity of the flow.

As mentioned previous chapter, the current using cavity mechanism has two kind of door

opening system; vertical and sliding. In order to see the effect of door on the pressure changing,

several combinations have been tried such as changing the opening time and the free-stream

velocity.

The free-stream velocity has been fixes at 2m/sec. In figures, the cavity at the beginning was

completely closed till 17.5th

seconds then it was opening related to opening time. Until 17.5th

seconds, the steady pressure measurements have been taken.


o at x/L=0.55 and free-stream velocity was 2m/sec.

45





In these three figures, at 17.5 seconds the cavity was opening. Interestingly, the peak value has

been determined at the same time which is 22.5th

seconds and was the same for both 3.8 seconds

and 40 seconds opening time. In order to identify a characterization parameter, the relaxation

time which is the time required to be open steady pressure level of the Cp.

46

As seen in figure 41, the relaxation time is nearly 70th

seconds for both 3.3 seconds and 40

seconds opening. Similarly, in figure 42, for 40 seconds opening the relaxation time again exists

on 70th

seconds, however, for 3.3 seconds opening, that time observed on 60th

seconds.

Differently, the relaxation time in figure 43, for both opening, is 80th

seconds. Comparing the

relaxation time till 30o degree and 60

o opening, for 60

o degree opening, it needs more time to be

steady state case.

Although, the time gap of the opening door between 3.8 seconds and 40 seconds, they nearly

have the same relaxation time. The cavity door does not affect the relaxation time up to which

angle it opens. The possible reason for that could be shown as the free-stream velocity is very

low. Similarly, the resonance effect on the steady case which as known as Rossiter modes cannot

be observed, as well because of very low speed. However, at 40 seconds opening door, it could

be observed that the effect of the angle of the cavity door.


o at x/L=0.55 and free-stream velocity was changing.

47


o at x/L=0.55 and free-stream velocity was changing.

That experiment was done only changed the free-stream velocity. In figure 44 and 45, the aiming

of the experiment was to understand the effect of the opening can be seen well. Comparing the

higher free-stream velocities, at 1.67m/sec, the relaxation time is longer at 40 seconds opening

than 5 seconds opening time. It is now quite understandable that the free-stream velocity affects

the relaxation time.

48

Figure 46 Sliding Opening Door at x/L=0.55, opening time is 4.5 seconds.

Figure 47 Sliding Opening Door at x/L=0.55, opening time is 10 seconds.

In that experiment, sliding door mechanics has been used and this was introduced in the previous

chapter. Comparing to the vertical opening door type cavity, their relaxation time is quite shorter.

The effect of the opening time is also in sight, like in 4.5 seconds opening, the relaxation time is

49

nearly 10 seconds longer than 10 seconds opening whether ignoring the fluctuating pressure after

60 seconds for 10 seconds opening. The causes of these fluctuations are environmental effect

like room pressure or temperature changes because the wind tunnel is open circuit and has very

low speed and also the room where the wind tunnel is not well isolated system. The same

relaxation time could be observed from other positions pressure measurements as well and it can

be found on the Appendix D.

As mentioned in experimental set-up section, 12 pressure taps has been used which are located

on the cavity floor in order to measure the pressure fluctuation on the cavity surface. The next

discussion would be about how the pressure gradient changes with along the cavity floor.

Figure 48 Vertical Opening Door at x/L=0.30.

50

Figure 49 Vertical Opening Door at x/L=0.80.

As seen in figures, even there is not any clear apparent difference between positions. However,

in the literature, the pressure fluctuations should be observed especially in the downstream of the

cavity. Moreover, as indicating in figure 4.33, after 50th

seconds, there is not even any

fluctuation. Theoretically, these kind fluctuations are not expected at very low speeds. For the

other position of the pressure tap data’s can be found on the Appendix section of the thesis.

4.4 Flow Control Experiment The flow control experiment has been progressed in order to supress the oscillation as seen in the

opening door and when the cavity door is on. In literature review, Rowley et al. [33] has been

tried similar experiment without door.

In this experiment, the cavity had the same L/D ratio; however, its dimensions were different as

mentioned in previous experiment. The free-stream velocity was 18 m/sec and the boundary

layer thickness has been calculated as 5mm. The cavity door was opening in 2 seconds.

51

In the below figure, the blue line refers that at 18 m/sec, the cavity doors were opened without

using any flow control equipment, and the red line indicates the cavity doors were opened and

the cavity had cylinder row on the upstream edge of the cavity. As seen from figures, the amount

of mass was decreased in the cavity which has flow control and in the steady case, the fluctuation

was decreased.

Figure 50 Opening Door with and without flow control equipment.

Figure 51 Fully Opened Door with and without flow control equipment.

52

4.5 Repeatability Tests The door opening experiment at some has been repeated for x/L= 0.55 and 0.80 configurations in

order to review the repeatability of the results. The results can be found in Appendix section. The

data which was obtained from repeatability tests were consistent and the data considerable were

repeatable.

Figure 52 Repeatability test of the door opening.

Figure 53 Repeatability test of the door opening.

53

The test has been repeated at each point 30 times (Appendix D). In all figures, there are still

pressure variations for the steady open cavity case. It is likely to be due to the very low speed

free stream velocity and the difficulty of keeping the room pressure at constant level.

4.6 Preliminary PIV Results The mean streamlines are displayed in figure 54 and 55 for cavity fully open with and without

flow control cavities and the free-stream velocity has been determined as 2m/sec and Reynolds

number was 1.4×103

based on the distance from leading edge of the palate to the cavity leading

edge.

The mean flow pattern in the fully open cavity, figure 54, clearly displays a recirculation bubble

in the rear part of the cavity and a shear layer growing to the point where it nearly spans the

whole back wall of the cavity. The recirculation pattern is around x/L=0.85 and y/L=0.5 is very

large. The maximum velocity of the recirculation was determined as 5% of the free stream

velocity. Because of the bad quality of the smoke, the expected recirculation on the front of the

cavity could not observe. The streamlines impinges on the back wall of the cavity proved by

mean flow diagram.

Figure 54 Streamlines from PIV, for fully open cavity without flow control. (L=320mm, 2m/sec)

Flow direction

54

The mean velocity for the flow control cavity flow experiment is displayed in the below figure.

The observed recirculation on fully open cavity without flow control was on the x/L=0.85,

although on flow control cavity, the recirculation bubble was observed x/L=0.60 and y/L=0.20.

In addition, the velocity of the bubble is around again 5% of the free-stream velocity. Comparing

the without flow control experiment, the shear layer flow on the flow experiment was shifted

above from the cavity as seen in the figure. As known from literature, the shear layer flow which

causes the pressure fluctuation due to impinging on the aft wall of the cavity. This effect can be

reduced using flow control equipment.

Figure 55 Streamlines from PIV, for fully open cavity with flow control (L=320 mm, 2 m/sec).

55

Chapter 5 Conclusion & Future Work

This thesis has prepared grounds for a better understanding of the transient cavity flow created

by movement of pivoting or sliding doors. Preliminary investigation of flow control device on

the transient phase was also experimentally investigated.

Fundamental differences have been observed with and without the doors and during the door

opening. During the door opening, the flow inside the cavity depends on the time to open the

door from 00 degree to 90

0 degree. The time required to reach the open cavity pressure level

from closed is called relaxation time. The determination of that time is for practical applications

on flying vehicles.

In the experiments, the two types of cavity doors have been used in order to observe see the

effect of door kinematics. As a conclusion, the relaxation time for the sliding door is

significantly shorter compared with the vertical opening door system.

The relaxation time has also been observed also for different opening door durations. For 3.8 and

40 seconds opening times, relaxation times are almost equal.

As far as, free-stream velocity is concerned, the only difference observed was at very low free-

stream velocity, 1.67m/sec, for which the relaxation time increased by 10 seconds.

The flow control experiment is encouraging, as the transient phase seems shorter while steady

pressure level is lower with the flow control device.

Recommendations for future work would include that the same experiments could be tried at

higher Mach number in order to compare the results with existing works for fully open cavity

and also to observe the emergence of Rossiter modes in transient phase.

56

Chapter 6 References

1) J.E. Rossiter, The Effects of Cavities on the Buffetting of Aircraft, Technical

Memorandum AERO.754, Royal Aircraft Establishment, April 1962.

2) M.B. Tracy, E.B. Plentovich and J. Chu. Measurements of Fluctuating Pressure in a

Rectangular Cavity in Transonic Flow at High Reynolds Numbers. Technical

Memorandum 4363, NASA, June 1992.

3) E.B. Plentovich, R.L. Stallings, Jr., and M.B. Tracy, Experimental Cavity Pressure

Measurements at Subsonic and Transonic Speeds, Technical Paper 3358, NASA, 1993.

4) K. Krishnamurty, Acoustic Radiation From Two-Dimensional Rectangular Cutouts in

Aerodynamic Surfaces, Technical Report Technical Note 3487, National Advisor

Committee For Aeronautics, August 1955.

5) J.E. Rossiter, A Preliminary Investigation into Armament Bay Buffet at Subsonic and

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Memorandum AERO.754, Royal Aircraft Establishment, April 1962.

8) J.E. Rossiter, A Note on Periodic Pressure Fluctuations in the Flow Over Open Cavities,

Technical Memorandum AERO.743, Royal Aircraft Establishment, November 1961.

57

9) J.E. Rossiter and A.G. Kurn, Wind Tunnel Measurements of the Unsteady Pressures In

and Behind a Bomb Bay (Canberra), Technical Note AERO.2845, Royal Aircraft

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Behind a Bomb Bay (T.S.R.2), Technical Report AERO.2677, Royal Aircraft

Establishment, August 1963.

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Physical Mechanisms and Suppression Concepts, Technical Report AFFDL-TR-74-133,

Air Force Flight Dynamics Laboratory, 1975.

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Rectangular Cavity in Transonic Flow at High Reynolds Numbers, Technical

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29 2005. NATO. Paper 10.

58

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Flows – Part I: The Pressure Field, Journal of the Aerospace Sciences, 28(6):457–470,

June 1961.

18) K. Atvars, K. Knowles, S.A. Ritchie, and N.J. Lawson, Experimental and Computational

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19) L. Ukeiley and N. Murray, Velocity and Surface Pressure Measurements in an Open

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20) D.A. Norton. Investigation of B47 Bomb Bay Buffet. Technical Report D12675, Boeing

Airplane Company, May 1952.

21) J.E. Rossiter. The Effect of Cavities on the Buffeting of Aircraft. Technical Memorandum

754, Royal Aircraft Establishment, Farnborough, UK, April 1962.

22) J.E. Rossiter and A.G. Kurn. Wind Tunnel Measurements of the Unsteady Pressures In

and Behind a Bomb Bay (T.S.R.2). Technical Note 2677, Royal Aircraft Establishment,

Farnborough, UK, August 1963.

23) L.L. Shaw and R.M. Shimovetz. Weapons Bay Acoustic Environment. In Impact of

Acoustic Loads on Aircraft Structures, Lillehammer, Norway, May 1994. RTO

Symposium. Also CEAS/AIAA-95-141.

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of Flow-Induced Cavity Oscillations, Progress in Aerospace Sciences, 2008.

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59

25) R.A. Smith, E. Gutmark, and K.C. Schadow, Mitigation of Pressure Oscillations Induced

by Supersonic Flow Over Slender Cavities, Journal of Aircraft, 29(6):999–1004,

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60

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61

Appendix

Appendix A. Calibrations

A1 Instrumentation Calibration

A1.1 Pitot Tube Calibration

The free stream velocity of the wind tunnel has been determined by using FC012 type Micro

manometer. Besides, during the unsteady and steady pressure measurements, the same

manometer has been used.

The reading has been recorded by digital screen and digital output via cable. The digital screen

gives the pressure in mmH20 unit and comparing the pressure unit with voltage, the linear

calibration curved has been obtained.

Figure A1. FC012 type Micro manometer.

62

Figure A2. The manometer pressure curve.

A1.2 Hot-Wire Calibration

For the investigations of the boundary layer, CTA types hot-wire has been used. From digital

output of that Dantec type hot-wire box, the data has been recorded using data acquisition card.

The calibration of the hot-wire has been done using pitot-tube. The curve of the calibration as

shown in the figure.

Figure A3. Hot-Wire Calibration Curve.

63

Appendix B. Software

B1 Labview The data acquisition or motor controller has been controlled by using Signal Express Labview

Programme. The structure of the programme is shown in the below figure.

Figure B1. Labview Signal Express Programme

By that programme structure, the pressure data, the position of the doors could be recorded and

also the servo motor can be controlled. The programme is able to generate digital and analog

signals in order to required conditions.

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B2 Calculations on Matlab close all clc, clear all load 18_07; alldata = load('18_07');

info = whos;

% for i = 1 : length(info) % info(i).name % end

j = 0; for i = 2 : 2 % info(i).name j = j+1;

ps = alldata.(info(i).name);

N = 1000; for k = 1 : ceil(length(ps(:,1))/N)

ps_av(k) = mean( ps(N*(k-1)+1:N*(k),2) ); ts_av(k) = mean( ps(N*(k-1)+1:N*(k),1) );

end

pressures= 3.9355*ps_av+0.0019;

cps=(pressures-2.3778)/(0.5*1.2*4);

% a=(cps./0.003079); figure(j) plot(ts_av,cps ,'-r','LineWidth',2) title ( ['Vertical Opening Door at ', ' x/L= 0.55' ]) % title ( [' x/L= 0.', info(i).name(3:4), ' repeat = ',

info(i).name(5:5)]) xlabel( ' time ' ) ylabel( ' Cp ' )

axis([ 0 97.5 -0.17 0.1]) grid on hold on

end % j = 0;

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for i = 3:3 info(i).name j = j+1;

p080 = alldata.(info(i).name);

N = 1000; for k = 1 : ceil(length(p080(:,1))/N)

p080_av(k) = mean( p080(N*(k-1)+1:N*(k),2) ); t080_av(k) = mean( p080(N*(k-1)+1:N*(k),1) );

end

pressure080= 3.9355*p080_av+0.0019;

cp080=(pressure080-2.3778)/(0.5*1.2*4);

% b= cp080./0.003079; figure(j) plot(t080_av,cp080,'--b','LineWidth',2)

hold on

end

j = 0; for i = 4 : 4 info(i).name j = j+1;

p5 = alldata.(info(i).name);

N = 1000; for k = 1 : ceil(length(p5(:,1))/N)

p5_av(k) = mean( p5(N*(k-1)+1:N*(k),2) ); t5_av(k) = mean( p5(N*(k-1)+1:N*(k),1) );

end

pressure5= 3.9355*p5_av+0.0019;

cp5=(pressure5-2.3778)/(0.5*1.2*4);

figure(j) plot(t5_av,mean(cp5),'.g','LineWidth',2) legend(' Opening time 3.8 secs ','Opening time 40 secs','steady pressure

')

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hold on

end

Appendix C. Drawings for Cavity The constructions and modifications of the cavity have been carried out using AutoCAD and

SolidWorks. According to necessities, the cavity has been enhanced for pressure measurements,

cavity door mechanics, and constructing a new cavity.

Figure C1. The leading edge for the cavity plate

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Figure C2. The cavity door for bigger cavity.

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Appendix D. Experimental Results

D.1.1 Repeatability Tests The repeatability test are shown in the below figures.

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o degrees.

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o degrees.

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o degrees.

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D1.4 Sliding Opening Door

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